Eruptions of the Yellowstone volcanic system have included the two largest volcanic eruptions in North America in the past few million years; the third largest was at Long Valley in California and produced the Bishop ash bed.
On the diesel Passat, the TDI badge stands for Turbocharged Direct Injection, which isn't really all that helpful as the gasoline-powered Passat is also turbocharged and also uses direct injection. Volkswagen attempts to clear this up by calling this Passat the Passat TDI Clean Diesel.
They are good cars, actually. The TDI cars have long been gems in the VW and Audi lineups. They provide good power, great fuel economy and loads of torque that happen to make them pretty fun to drive. I was thoroughly impressed by the 2015 Golf SportWagen TDI I tested earlier this year.